(a) Field of the Invention
Broadly speaking, this invention relates to signal transmission systems. More particularly, in a preferred embodiment, this invention relates to signal transmission systems in which the distortion produced by non-linearities in the operation of an active device is substantially eliminated through the introduction of a compensating, distortion-cancelling signal.
(B) Discussion of the Prior Art
Pre-distortion and post-distortion techniques for cancelling the distortion introduced by the non-linear transfer characteristics of active devices, such as amplifiers, are well known. In a typical prior art arrangement, for example, as discussed in U.S. Pat. No. 3,383,618 which issued on May 14, 1968 to R. S. Engelbrecht, a non-linear device in a compensation circuit is driven by a portion of the output signal of an amplifier. The non-linear device generates a composite signal containing a host of distortion components covering a range of multiple orders of distortion. All of these distortion components pass through two controllers, one for phase and the other for amplitude, before they are coupled with the output signal of the amplifier to provide a reduction in overall signal distortion through complementary cancellation. In the above and other similar arrangements, it is necessary to adjust the phase and amplitude of all of the distortion components as a single composite signal to eliminate the third-order distortion and thereby obtain an overall reduction in signal distortion.
Whenever the signal bandwidth is small compared to the center frequency, the even orders of non-linear distortion fall outside the signal band and only certain odd orders of distortion fall inside the signal band. Of these odd orders of distortion the third order is likely to be the largest and most troublesome. The higher orders of distortion, that is orders greater than the third, which are present in the output of an uncompensated bandpass amplifier, are usually small. The higher, odd-order distortion components present in the output of the non-linear device which is used to compensate the amplifier have a different phase and amplitude than the corresponding higher orders of distortion in the output of the amplifier. These differences are due to unavoidable minute deviations between the characteristics of the compensating non-linear device and the amplifier. Therefore, when these two outputs are combined, the third-order distortion may be reduced, but the higher orders of distortion are typically magnified. This disadvantageous compromise renders many prior art distortion compensation techniques ineffective for numerous applications. Such compensation techniques are particularly inadequate for use in analogue transmission systems which employ, in tandem, numerous repeater amplifiers in the transmission path.
To overcome the above problem, the distortion compensating circuit disclosed in commonly-assigned U.S. Pat. No. 3,825,843, which issued on July 23, 1974 to R. I. Felsberg and H. Miedema, was developed. In this circuit, third-order distortion in a signal path is substantially eliminated without a detrimental increase in higher orders of distortion. A portion of the signal in the signal path is extracted and applied to a squarer and a multiplier which together comprise a compensation circuit. The squarer operates on its input signal to produce a second-order output signal. In the multiplier, the second-order output signal and the other input signal thereto are multiplied together to produce a third-order output signal. The phase and amplitude of the third-order signal are adjusted to provide a compensating signal. This compensating signal is then coupled to the signal path so that the third-order distortion produced in the signal path is substantially eliminated through complementary cancellation.
Although the circuit disclosed in the above-referenced U.S. Pat. 3,825,843 was successful in eliminating third-order distortion, the interaction between this circuit and the non-linear circuit it is intended to linearize introduces higher orders of distortion not originally present in the output signal. These higher order distortion components are, however, relatively low in amplitude when the output level is well below saturation and do not pose a problem in most applications. However, the level of these higher order components increases rapidly with increasing signal levels and the trend to even higher output levels can reach the point where these higher-order distortion components may no longer safely be ignored.
Accordingly, as taught in the U.S. Pat. No. 4,016,497, if the distortion compensating circuit shown in U.S. Pat. No. 3,825,843 is modified such that it operates in a feedbackward mode rather than a feedforward mode, the third-order distortion terms are eliminated, as before, but without the generation of additional higher order distortion terms.
Unfortunately, even the improved distortion cancelling circuit disclosed in the referenced co-pending application of Miedema has its drawbacks. More specifically, the circuit is relatively expensive to manufacture, due to the cost of the cuber, phase resolver, amplifier, etc., and the initial adjustment of the device is fairly complex, due to the need to adjust the delay of the separate circuit branches.
An article entitled "Reseau lineariseur pour tube a onde progressive" (Linearizing System for Travelling-Wave Tubes) by C. Bremenson and P. Jaubert, Revue Technique Thomson CSF, Vol. 6, No. 2, June 1974, pp. 529-548, discloses a complementary distortion circuit which employs a varistor arrangement as the non-linear element in a bridge circuit. As shown in FIG. 9 of the Bremenson and Jaubert article, the complementary distortion circuit, designed to operate at 6 GHz, includes a 3 db -- 90.degree. hybrid coupler, a 3 db -- 180.degree. hybrid coupler and a non-linear varistor circuit in the lower branch. This arrangement is intended to maximize the output power of a travelling wave tube, given that the level of intermodulation products should be at least 25 db below the level of the fundamental signals of the TWT output.
Unfortunately, this arrangement does not solve the problem with which applicants were faced -- namely to reduce the level of IM products at a TWT output by 25 db, a far more difficult requirement and one which cannot be solved unless the effects of gain compression/expansion and capacitive/inductive AM/PM conversion are simultaneously accounted for.